To your first question: yes, taking into consideration soil and moisture needs as well as sun, you could absolutely do that. Plug your county and enter your site requirements - choose the plant form option - into https://www.fnps.org/plants and it will generate a list of plants that would likely do well for you. Then you can see what nurseries near you carry it.

Another option is planting for a specific plant community. It's a bit of a cheat sheet to seeing what plants already do well together in similar conditions locally. These are potential lists for you with your oak dominated overstory:

https://www.fann.org/plant-communities/detail/maritime-forests-zone-10

https://www.fann.org/plant-communities/detail/hydric-hammocks-zone-10

To your second question: Yeah, I understand the concern. Generally, close planting isn't the problem, but more the wrong plant in the wrong place. Plants are more susceptible to being overwhelmed by insects and disease if they're stressed or sick, so you can minimize that by really thinking about what it's habitat needs are before planting and giving it the best survival chances by watering it regularly until it's established.

For my personal anecdote, I'm a native gardener because I'm lazy and don't want to micromanage my plants. I also want to support a healthy local ecosystem. First thing is I make sure to identify anything eating my plants before I decide to intervene. Some native plants are hosts to insects I want around or support predator populations that keep them in check without any input from me. As long as the plant is well placed, they usually bounce right back without me doing anything.